In the powder to be mixed into the cosmetics, the primary particle size ranges from nano order to around a few to several hundred microns, and the powder exists in the form of aggregate of particles of primary particles. The primary particle sizes of the powders are determined by the chemical composition and a range of the particulate sizes of that powder under consideration of a function to be exhibited in the cosmetics. For example, in the case of titanium dioxide, in order to improve the usability and protection from infrared rays, the primary particle sizes are set to particle sizes of several micrometers in a geometrically optical range; in order to exhibit a shielding power and a coloring power, they are set to particle sizes of near half wavelengths of visible light rays, that is, Mie scattering range; and in order to exhibit an ultraviolet ray shielding power, they are set to particle sizes of a Rayleigh scattering range considerably smaller than the wavelengths of the visible light rays.
As the powder particles are more dispersed in a state nearer to the set primary particle sizes, the functions possessed by the powder, that is, for example, characteristics sought for the cosmetics, such as the shielding effect, the coloring effect, an ultraviolet ray scattering effect, transparency, optical characteristics, skin adhesion, touch to skin and the like, can be exhibited to the utmost extent. However, the smaller the primary particle sizes of the powder are, the greater is the specific surface area, so that since the particles have increased contact points therebetween, the particles are more likely to be aggregated and consequently the dispersability is deteriorated. A number of surface-treating techniques have been proposed as certain measures to avoid this aggregation as much as possible, and prevent re-aggregation even with the lapse of time.
There are proposed a variety of surface-treating methods, for example, (1) a method in which a silicone oil (for instance, methyl polysiloxane, methyl hydrogen polysiloxane or alkyl silane with the number of carbon atoms of an alkyl portion being not more than 10) is dissolved into a solvent as a surface-treating agent, which is added and mixed into a powder, and the surface treatment is baked onto the powder by heating after the drying process; (2) a method in which, while a powder and octyl triethoxy silane or the like are being dispersed into an organic solvent by using a media grinder, the surface of the powder is treated with an organic silicon compound such as octyl triethoxy silane (Patent Document 1); (3) a method in which N-octyl trimethoxy silane or N-octyl triethoxy silane as an alkyl silane compound is stirred and mixed by a Henschel mixer, and a reaction is completed under heating, and the resultant is pulverized by a hammer mill (Patent Document 2); (4) a method in which a silicone compound such as methyl hydrogen polysiloxane or the like is emulsified by dispersing it in water, and surfaces of powder particles are coated by mixing the emulsion to the powder (Patent Document 3); (5) a jet method in which after a metal soap, an organic silicon compound in which a reactive group such as a hydrogen group or the like is bonded to a silicon atom, and a powder are mixed, the mixture is pulverized by a miller using an ejecting stream simultaneously with the surface treatment (Patent Document 4); and (6) a method in which in order to improve dispersability of a powder, coating is effected with surface treating agents for an A layer and a layer B by a jet method (Patent Document 5).